Compact optics assembly for a QKD station
View Patent ↗A bulk-optics assembly for a transmitting/receiving QKD station (BOB1) in a two-way autocompensating QKD system ( 101 ) is disclosed. The assembly consists of a first beamsplitter ( 104 ) having a high (e.g., 90:10) beamsplitting ratio, a 50:50 beamsplitter ( 106 ) and a polarizing beamsplitter ( 108 ). The assembly also optionally includes a polarizer ( 102 ), and/or a fixed attenuator (FOA), and/or an optional blocking filter ( 110 ) downstream of the polarizing beamsplitter. The compact bulk-optics assembly is easier to manufacture than a fiber-based optical system, and is simpler and more compact than prior art bulk-optics assemblies for QKD systems.
1. An optical assembly for a QKD station, consisting of, in order along an optical axis:
a polarizer;
a first beamsplitter having at least a 90:10 beamsplitting ratio;
a second beamsplitter having a 50:50 beamsplitting ratio and arranged to receive light that passes through the first beamsplitter along the optical axis; and
a third beamsplitter that is a polarizing beamsplitter and that is arranged to receive light that passes through the second beamsplitter along the optical axis.
2. The assembly of claim 1 , further consisting of a fixed attenuator arranged immediately adjacent the polarizer, wherein the fixed attenuator has an attenuation of between about 10 dB and 20 dB.
3. The assembly of claim 1 , wherein the first beamsplitter has at least a 99:1 beamsplitting ratio.
4. The assembly of claim 1 , further consisting of an optical filter arranged immediately downstream of the third beamsplitter.
5. The assembly of claim 1 , further consisting of:
a first port adapted to input light from a laser into the assembly at the polarizer via a first optical fiber section coupled to the first port and to the laser;
a second port coupled to the second beamsplitter, with the second port coupled to a second optical fiber section that is coupled to a phase modulator;
a third port coupled to the third beamsplitter, with the third port coupled to a third optical fiber section that is coupled to the phase modulator; and
wherein the second and third optical fiber sections are arranged such that light having a polarization and entering or leaving the second port via the second and third optical fiber sections is rotated by 90° relative to light entering or leaving the third port.
6. The assembly of claim 5 , further including:
a first detector port coupled to the second beamsplitter, with the first detector port coupled to a fourth optical fiber section that in turn is coupled to a first single-photon detector; and
a second detector port coupled to the first beamsplitter, with the second detector port coupled to a fifth optical fiber section that in turn is coupled to a second single-photon detector.
7. The assembly of claim 4 , further including a housing that contains the polarizer, the first, second and third beamsplitters, and the optical filter, and wherein the housing has dimensions of about 3″×0.6″×0.6″.
8. The assembly of claim 1 , wherein the polarizer and the first, second and third beamsplitters include an antireflection coating designed to minimally reflect a quantum signal wavelength.
9. An optical assembly for QKD station, consisting of, in order along an optical axis:
a polarizer;
a first beamsplitter having at least a 90:10 beamsplitting ratio;
a second beamsplitter having a 50:50 beamsplitting ratio and arranged to receive light that passes through the first beamsplitter along the optical axis;
a third beamsplitter that is a polarizing beamsplitter and that is arranged to receive light that passes through the second beamsplitter along the optical axis; and
a housing that contains the first, second and third beamsplitters.
10. The assembly of claim 9 , wherein the polarizer is contained within the housing.
11. A QKD station comprising:
a compact optical assembly consisting of a polarizer, a first beamsplitter having at least a 90:10 beamsplitting ratio, a second beamsplitter having a 50:50 beamsplitting ratio, and a third beamsplitter that is a polarizing beamsplitter;
a laser adapted to emit pulses of light and optically coupled to the polarizer;
a phase modulator optically coupled to the second and third beamsplitters with an optical fiber arranged such that light having a polarization and entering or leaving the second beamsplitter and traveling through the phase modulator is rotated by 90° relative to light entering or leaving the third beamsplitter.
12. The QKD station of claim 11 , further including:
a first single-photon detector optically coupled to the second beamsplitter; and
a second single-photon detector optically coupled to the third beamsplitter.